Improved techniques for the high speed blade coating of paper



May 1963 P. 1'. KUHNEL 3,088,842

IMPROVED TECHNIQUES FOR THE HIGH SPEED BLADE COATING OF PAPER Filed May 11, 1959 2 Sheets-Sheet. 1

COATING WEB COATING BLADE WEB COATING P. T. KUHNEL May 7, 1963 IMPROVED TECHNIQUES FOR THE HIGH SPEED BLADE COATING OF PAPER Filed May 11, 1959 2 Sheets-Sheet 2 United States Patent C) Delaware Filed May 11, 1959, Ser. No. 812,191 3 Claims. (Cl. 117-7) This invention relates to improvements in the coating of paper or the like, and it particularly relates to a method incorporating improved techniques for the high speed blade coating of paper in a manner substantially to improve the surface texture and printability thereof.

An object of the invention is to provide a method which involves improved techniques for the blade c oating of a continuously moving web at high speed in a manner productive of excellent surface texture and superior printability.

A still further object is to provide improved operating techniques which permit existing coating equipment to be structurally modified in the manner herein taught for the high speed production of a higher quality publication grade or book paper than has heretofore been produced on existing equipment at the speeds involved.

Another object is simultaneously to lower the cost and raise the quality of publication grade or book paper by employment of the improved method and apparatus herein taught.

Other objects will be in part obvious and in part pointed out in more detail hereinafter.

The invention accordingly consists in the features of construction, the combination of elements, and particularly in the arrangement thereof as exemplified in the drawings and in the adjustment of the elements to establish operating conditions as taught herein within parameters as defined in the appended claims.

In the drawings,

FIG. 1 illustrates diagrammatically a coating apparatus useful in the practice of the method of the invention,

FIG. 2 is a fragmentary enlarged section showing a web supported by a backing roll while being blade coated,

FIG. 3 is a greatly enlarged fragmentary section, similar to FIG. 2 but forward of the blade and illustrating the paper as coated in the manner taught herein,

FIG. 4 is an enlarged fragmentary cross-sectional detail of the applicator nip-forming rolls shown in FIG. 1, and

FIG. 5 shows diagrammatically another embodiment of coating apparatus useful in the practice of the invention.

It is well understood that while structurally the various types of apparatus heretofore devised for the application of coating to fibrous webs such as paper appear quite simple, the art itself is technically quite complex. Many basic discoveries have resulted from structural modifications of production apparatus, which when first viewed in retrospect often appear to be only slight improvements, since many of the structural components involved are common to those employed in other coating methods. A careful study is frequently required for an understanding of the manner in which slight structural modifications and operating conditions often lead to the elimination of manufacturing bottle-necks and to a substantially improved end product. Examples of such advances now generally regarded by the industry as repre-,

senting basic changes in coating technique are the Kimberly-Clark-Mead r011 coater and the pond type blade coater such as are illustrated in the article Paper Coating Trends in the United States, beginning on page 389, of

ICC

the June 30, 1958, issue of Svensk Pappers Tidning (Swedish Paper Journal).

While each of the above methods of coating oifer certain advantages, production-wise, depending upon the particular operating conditions and product desired, need has arisen for an improved blade coater which is capable of functioning effectively at web forming speeds which were unattainable only a few years back, but which are now commonly obtained by papermaking machinery of advanced design. The present invention fills that need in an effective manner while making it possible, by modification of costly roll coating apparatus now installed in many plants, to incorporate the inventive concepts herein taught. The required modification of existing equipment for incorporation of the present invention, while costly, represents a very substantial saving as compared to the cost of installing known blade coaters.

FIG. 1 illustrates the invention incorporated in apparatus, the major components of which previously may have been utilized for the roll coating of paper by the well known Kimberly-Clark-Mead method as illustrated in the above mentioned article. In the practice of that coating method a pool of coating 10 is maintained in a nip defined between a pair of counter-rotating metering rolls 12 and 14 for transfer of a uniform layer of coating first to the surface of roll '14, then'to the surface of an applicator roll 16. The coating thus transferred moves, with roll 16, into an applicator nip 17 defined between the applicator roll 16 and a backing roll 18, the latter roll being driven in counter-rotation to roll 16 for transfer of the coating to a paper web shown dotted at 20 as normally positioned for roll coating. Web 20 is fed through the applicator nip with little or no wrap around the surface of backing roll 18 and, of course, none in respect to applicator roll 16.

Since in roll coating apparatus the sole purpose of feeding web 20 through the applicator nip is to effect pressure transfer of a major portion of the uniform layer of coating from the applicator roll surface to the web, there being a split at the outgoing side of the nip as well understood in the art and shown in FIG. 4, the coated web is normally drawn from the outgoing side of the applicator nip at about the dotted position shown. No useful purpose would be served in wrapping the web thus roll coated around the backing roll, unless apparatus design requires a mere change in direction of the web after being coated. The investment in apparatus of this type by major US. producers of book or publication grade paper is very substantial, and complete replacement of such equipment for conversion to blade coating, as above stated, costs substantially more than the cost of structural modifications required for incorporation of the present invention in costly existing equipment. 8

The invention herein taught makes possible the utilization of metering rolls 12 and 14, applicator roll 16, and backing roll 18 substantially as they are currently installed for the above mentioned roll coating. Certain adjustments within new ranges are required, the design of most modern papermaking machines being such that the adjustments can be obtained without alteration of the structure. Additionally required modifications may in most instances be made at a comparatively low cost, as may the few new components to be added.

It is now generally conceded that for certain printing requirements blade coated paper is superior to roll coated paper. Certain production limitations heretofore believed inherent in the blade coating processes, especially those encountered at very high operating speeds, have prevented a lowering of production costs of publication grade paper, since manufacturers have been unable to obtain the required high quality when operating within the maximum speed ranges of papermaking machines of the most advanced design. The modern papermaking machine, which represents a very large capital investment, is capable of producing a high quality fibrous web, suitable for use, when properly coated, as high grade publication paper, at speeds substantially greater than known blade coaters can apply coating thereto while maintaining surface texture and printability at the very high quality levels which the printers demand.

The industry has directed a substantial arnount of engineering and research effort toward the elimination of the above mentioned inherent production problems, but the solution has heretofore remained elusive. Satisfactory operating speeds have not been consistently maintained without sacrifice of the quality of the end product. Hence the highest quality publication grade paper has heretofore been produced at speeds lower than the maximum speed of advanced design machines.

Blade coaters to which the industrys attention is currently directed are of two types, i.e., the pond type blade coater wherein the coating is first applied to the Web by immersion and the type wherein the coating is applied to the web by roll applicator means. In the pond type coater the blade forms one wall of a trough, the other side of which is formed by the web to be coated as it moves around a backing roll with the blade edge serving also as the part of the applicator nip. The latter type coater employs a blade circumferentially spaced forward of the outgoing side of an applicator nip to engage the web as supported by the backing roll for the removal of an excess portion of the coating transferred to the web from the surface of the applicator roll during web travel through the applicator nip. The present invention is not concerned with the pond type coater, but rather is directed to improvements, both in respect to the structure and operating techniques, of blade coaters of the latter general type with particular reference to an onthe-machine coater effectively operable throughout the current range, of paper making machine speeds.

Referring to the apparatus of FIGS. 1 and 5 which illustrate identical components, a paper web 22 to be coated in accordance with the present invention is fed from a drier section of the paper machine, assuming continuous on-the-machine operation of the coater, but for convenience of illustration is shown fed from'a supply roll 24 as during off-the-machine operation. Roll 24 is maintained under frictional drag or back tension the optimum value of which may vary widely for different runs but which is predetermined and fixed for each set of operating conditions by a known type of adjustable tension device 26 which may be associated with a drier roll for on-the-machine operation. Web 22 travels to a lead roll 28, and then to the surface of backing roll 18 through. a path which insures at least a minimum of 45 of wraparound the surface of roll '18 as shown in FIG. 5 before web 22 arrives at the center of applicator nip 17. The required wrap varies with the weight or strength of the web to which a mineral content coating is to be applied, a 45 wrapbeing the minimum wrap for heavy strong webs.-

Best results for all but the heaviest webs are obtained byemploying at least a 60 Wrap as shown in FIG. 1, while a 90 wrap may be required for very light webs, the relation between web weight and required wrap being explained below. While the backing and applicator rolls may be driven to rotate at a like circumferential speed it is preferred that the backing roll be driven at a slightly higher speed than the applicator roll. For example, it has been found that under one set of operating conditions good results were obtained by a backing roll surface speed of 900 feet per minute and an applicator roll surface speed of 820 feet per minute. The slight speed differential between rolls minimizes forces which might tend to oppose the substantial inherent tension under which the web must be maintained both during initial application of the coating thereto and during the subsequent 4 blading operation. After passage through nip 17, web 22 continues its wrap around the surface of backing roll 18 to pass under a coating blade 30 while still in snug contact with the backing roll surface due to contractile bias of the pre-tensioned web as it continues its wrap around roll 18 for at least a short circumferential distance after passage under blade 30 to prevent fluttering. The Web may be removed from the surface of roll 18 at any convenient position thereafter to pass around a guide roll 32, then through a suitable drying section, illustrated by a can drier 34, and eventually to a driven windup roll 36. Web 22 is thus pulled forward by and at the circumferential speed of driven roll 18 against the frictional resistance of the components which supply and direct the web to roll 18 to which is added by device 26 sufiicient additional resistance to place the web under considerable inherent tension. Wind-up device 36 places web 22, after being coated, under sulficient tension to insure tautness during wind-up. The various rotating components of the system are driven by any suitable means, such as driving motors 50, 51, 52 and 53 for the rolls 16 and 18, the drier 34 and the windup roll 36.

Frictional resistance between the web and the backing roll surface established by the web as pretensioned is sufficient to prevent relative movement therebetween while insuring positive drive of the web by the roll.

While preferably both applicator roll 16 and backing roll 18 are resiliently surfaced, it is mandatory that at least backing roll 18 be resiliently surfaced as by having applied thereto a rather stiff rubber-like covering 38, FIG. 2, to allow some yielding under the pressures applied thereto both within thenip area and under the coating blade. The resiliency of the backing roll surface is preferably in the range of 30 to P and I, using a /s" ball. The P and J (Pusey and Jones) hardness measuring system is, for example, described in Tappi for September 1953, pages 120-A and l2l-A (published monthly by the Technical Association of the Pulp and Paper Industry at 20th and Northampton Streets, Easton, Pa.), and in Tappi for July 1952, pages 309 to 311. Coating blade 30, the circumferential position of which controls the dwell time for a given circumferential speed, as later described, trails the center of applicator nip 17 by a minimum of 45, as shown in FIG. 1, but is preferably positioned at about 75 from the nip under most operating conditions, as shown in FIG. 5. The optimum position of blade 30 is governed by not only the speed of the backing roll, but also by the physical characteristics of both the web and the coating applied thereto. Blade 30 is obliquely disposed to a tangent to the surface of roll 18 at the line of blade contact, which may vary Within a range of 40 to 60.

The inventive concepts incorporated in the structures of FIGS. 1 and 5 which permit the coating of high grade publication paper at consistently higher speeds than have heretofore been obtained require detailed explanation, since the critical operating ranges might not be clearly understood from the diagrammatic illustrations of the structure. The minimum extent to which web 22 is wrapped or snubbed, while under predetermined tension, around backing roll 18 prior to movement into the center of the applicator nip is a function of many variables, such as web weight, moisture content, viscosity, solids content and quantity of the applied coating, the applicator nip pressure, and the dwell time between particular operating conditions. required minimum has no effect on coating quality, hence where permitted by the design of an existing machine or in equipment designed to incorporate the invention it is preferred that a wrap of or more be employed to cover all operating conditions, since very light weight webs may require a wrap of 90 or slightly more. The applicator nip pressure must also be maintained within a critical range, as must the drag or back tension as applied by device 26. Both the optimum nip pressure and the A wrap in excess of the nip and blade for optimum back tension are a function of the physical characteristics of the web, the type, solids content, viscosity and quantity of coating applied, the operating speed, and the blade position and 'angularity to a tangent at the line of web contact.

FIG. 1 shows applicator roll 16 and backing roll 13 with the axes thereof disposed obliquely to a vertical plane with the center of the applicator pressure nip between the 7 and 8 oclock position in respect to roll 18', as viewed. Whether the two rolls are positioned as in FIG. 1, or mounted in a common vertical plane, as shown in FIG. or otherwise positioned is a matter of design choice. Existing equipment may include corresponding rolls either vertically mounted or obliquely mounted to some extent, but in either case lead roll 28 is so positioned as to insure not less than the minimum 45 wrap, and preferably a substantially greater wrap of web 22 around the resilient surface of backing roll 18' prior to web passage through the center of applicator nip 17'.

As above mentioned, it is common roll coating practice to feed a web into an applicator nip with little or no wrap around either of the rolls forming the nip. Nip pressures employed for roll coating are commonly in the range of 50 to 60 pounds per lineal inch, and merely enough drag or tension is maintained on web 20 to insure proper web feed into and through the nip. Design factors frequently require that a web being roll coated be partially wrapped around a backing roll prior to passage through an applicator nip. However such a web is not subjected to an appreciable degree of inherent tension, nor would benefits result if the web were maintained appreciably tensioned since the problems solved by the present invention are obviously absent in roll coating techniques which involve high nip pressures with the coating operation completed within the applicator nip rather than by a blade following a dwell period. Attempts to modify.

such roll waters for blade coating by merely directing the outgoing coated web partially around the backing roll and under a pressure blade mounted circumferentially spaced from the outgoing side of the nip have fallen short of desired performance for the following reasons. As some of the fluid content of the viscous coating material quickly permeates the fibrous web under applicator nip pressures within the roll coating range the web increases in length, andsince a pressure line exists both along the center of the nip, and along the blade edge, the web becomes slack between those pressure lines. A coating of a surface smoothness which insures excellent printability can be obtained with the web held firmly snug against the backing roll during coating transfer thereto within the applicator nip as well as during the dwell time and passage under the blade. The invention insures that the web, under all operating conditions, is held taut between those spaced pressure lines resulting from the nip and blade regardless of web growth therebetween, thus insuring the continuous high speed production of a uniformly high quality paper.

To insure tautness between the above mentioned pressure lines, the web is first subjected to a predetermined substantial amount of inherent tension and in that condition is maintained sufficiently snubbed around the backing roll to retain that tension upon entry to the pressure nip. As the coating is applied under controlled pressure, that inherent tension, which is independent of the constantly applied back tension, since isolated therefrom by frictional forces resulting from the snubbing or wrap around the backing roll, compensates for both fluid growth of the web and for any looseness which might otherwise result from the high frictional resistance of the blade as itopposes the forward pull applied to the web by the wind-up roll. The invention thus insures, under all operating conditions including wide variations in web weights and types of coatings, proper snugness of the web against the backing roll surface in the area between the pressure line at the nip center and the line under 6 the edge of the blade. A minimum dwell time is required following web coating at the applicator nip and prior to blading to insure continuance of fluid migration following pressure application of the coating within the nip. While the dwell time between nip and blade is a function of the backing roll speed, a minimum permissible dwell time at the high web speeds involved is obtained by positioning the blade by at least a 45 are from the outgoing side of the nip. Thus the minimum permissible total web wrap around the backing roll is 90 which is obtained by adding the 45 minimum required wrap for pre-tensioning to the minimum permissible dwell angle of 45.

As above mentioned, attempts to employ roll coating techniques followed by blading have failed to produce high quality coatings at high speeds. Roll coating nip pressures of 50# to 60# per lineal inch result in a high degree of web penetration by the fluid content of the coating and a resultant immobilization of the adjacent solids content which does not blade in a manner to result in a surface finish comparable to that obtained by blading a coating while in the mobile state. By a reduction of the applicator nip pressure from the roll coating range of 50# to 60# to a range from about 2# to about 20# per inch the pressure induced migration of the coating fluid content is accordingly reduced to the extent that the adjacent solids content is applied to the surface of the web in the form of a mobile layer of a consistency to be partially removed by the blade while leaving a surface finish of very high quality and printability, It is important that a positive though relatively light nip pressure he employed. Attempts to employ a non-pressure applicator nip have met with no more success than have the above mentioned attempts to employ the higher roll coating pressures. This is believed due to the necessity of establishing an optimum degree of pressurized fluid migration into the web during coating transfer to the web within the nip area. Fluid migration must continue for a short time thereafter to insure that the interstices of the web are filled prior to blading since if the blading is done too quickly such migration following blading results in localized settling throughout the coated surface which while of relatively minor physical magnitude adversely affects the printability characteristics. Nip pressures within the present range result in an operation which is less time dependent than systems employing a non-pressurized nip while permitting blade positioning at a minimum dwell angle from the outgoing side of the nip.

Webbing of the type employed in publication grade paper normally ranges in basis weight between 25 and 60 pounds per 3300 square feet, and when pre-tensioned and snubbed around the backing roll to the extent of at least 45 for the heavier weight papers and as much as 90 or more for the lighter weights will retain sufiicient inherent tension to compensate both for wetness expansion and for any web slackness resulting from the forward travel of the web against the relatively high blade resistance above mentioned. It is important, however, that the rolls forming the applicator nip be adjusted for operation at hip pressures within the above mentioned range of about 2# to 20# per lineal inch. Substantially higher nip pressures result in too much coating permeating the web prior to the blading operation.

The web is preferably subjected to a back tension or drag force considerably in excess of that employed in roll coating techniques, and may vary from about 60% to of the web strength. The weight and other characteristics of the web will govern to some extent the force required to tension the web sufficiently to absorb web growth from the above mentioned causes while retaining sufficient residual tension to insure snugness between the web and backing roll during web travel from the nip to the blade which has been found very important in the prevention of wrinkles or uneven passage under the blade, especially at very high speeds. The extent to which the web must be snubbed around the resiliently surfaced backing roll is of course related to the frictional resistance between the particular web and the roll surface. The minimum permissible snubbing which is required to insure isolation of that portion of the inherently tensioned web between the nip and blade edge from the back tension or drag, is, as above mentioned, a 45 arc peripherally of the backing roll. While backing rolls vary in diameter and the peripheral are for a given angular sector increases accordingly, the arc flattens as the diameter increases and as well understood in the art, the frictional resistance which controls the snubbing action of a wrapped Web remains fairly constant in the range of roll sizes normally employed. The above minimum snubbing arc applies regardless of speed throughout normal operating ranges.

With the blade trailing the nip by at least the 45 mini mum angle, fluid migration into the web prior toblading is usually suflicient to insure good results, although highly viscous coating materials may require a somewhat greater dwell time which requires a larger angle for the same speed. The application of coating to a pretensioned web, followed by at least the minimum dwell period prior to blading results in a coated surface of a quality and printability far superior to that obtained when the same web is coated while under no appreciable tension. While not fully understood, those advantages add to the above stated advantages of maintaining the web taut against the backing roll between the above defined pressure lines to result in a very high quality printing surface.

While the drawings and description are directed to the single side coating of a continuously moving web, the concepts herein taught are of course equally applicable to the double coating of webs. For example, existing double roll coaters may obviously be modified to incorporate the teachings herein, the resulting structure being merely aggregative of the present invention, hence such a coating apparatus has not been specifically illustrated or described. Since successful practice of the invention is not dependent upon the type or physical characteristics either of the paper or the coating applied thereto, or to weight ranges of the finished product, the inclusion of specific examples of coating compositions, weights, etc. has been avoided since no useful purpose would be served thereby in teaching the invention, the scope of which is defined in the following claims.

I claim:

1. In a process for the production of coated paper in which a traveling paper web to be coated is continuously. drawn on a resiliently surfaced backing roll from a first pressure nip formed between the resiliently surfaced driven backing roll and a driven coating applicator roll to a second pressure nip formed between the said backing roll and a flexible blade, the steps of:

(a) driving the said backing roll and said coating applicator in counter rotation and in the direction of web travel;

(b) tensioning the said traveling paper web as it is passed to the backing roll to the extent of about 60-80% of the web strength;

() wrapping the said web while so tensioned around the surface of said backing roll to the extent of at least a 90 circumferential are;

(d) applying with said applicator roll an excess of a viscous coating to said Web at said first pressure nip at a position following the position of web application to said backing roll surface by a circumferential arc of at least 45 to form a uniform layer of coating on the web surface to force migration of the fluid contents of the coating into the web and to consequently expand the web lengthwise;

(e)- pressing the driven backing roll and driven applicator roll together to form said first pressure nip 8. under a light positive pressure while applying said coating to said tensioned traveling Web; and

(f) thereafter removing the excess portion of the applied coating by passing said Web on said backing roll to said second pressure nip formed by said flexible blade and said backing roll at a position following that of the coating application by a circumferential arc of at least 45 2. In a process for the production of coated paper in which a traveling paper web to be coated is continuously drawn on a resiliently surfaced backing roll from a first pressure nip formed between the resiliently surfaced driven backing roll and a driven coating applicator roll to a sec ond pressure nip formed between the said backing roll and a flexible blade, the steps of:

(a) pressuring the applicator roll and backing roll into line contact at a pressure of between about 2 to 20 pounds per lineal inch to form said first pressure p;

(b) drawing said tensioned web onto said surface of said backing roll at a position at least 45 prior to the said first pressure nip;

(c) drawing said Web from said first pressure nip and on the said backing roll through the second pressure nip so that said web passes on the said backing roll through a circumferential arc of at least 45 between the first pressure nip and second pressure nip;

(d) applying with the said applicator roll an excess of coating to the web on the backing roll at the said first pressure nip;

(e) blading off the excess of coating from the web at the second pressure nip; and

(f) tensioning the said web to the extent of between about 60-80% of .web strength as the web passes onto the said backing roll.

3. The method of coating a surface of a continuously traveling web of paper comprising applying a uniform layer of a viscous coating material to the surface of one of a pair of resiliently surfaced counter-rotating rolls positioned to define a pressure nip therebetween, stretching the web, snubbing the web of paper while maintained stretched to about of its elastic limit, around about one quarter of the circumference of the other roll on each side of the nip in the direction of the peripheral velocity of said rolls, maintaining the nip at an operating pressure of between 2 and 20 pounds per inch to effect pressurized transfer of coating from the first roll surface to the Web to cause web expansion lengthwise of the web, pressure blading the snubbed coated web at a position near the end of roll engagement to force a portion of said coating further into the web while removing an excess thereof to provide a smooth surface of high printability, and withdrawing the web from the second pressure nip and the backing roll.

References Cited in the file of this patent UNITED STATES PATENTS 291,628 Sackett Jan. 8, 1884 516,932 Hotchkiss Mar. 20, 1894 1,302,352 French Apr. 29, 1919 1,949,237 Bradner Feb. 27, 1934 2,257,373 Fanselow Sept. 30, 1941 2,285,531 Rhodes et al June 9, 1942 2,970,564 Warner Feb. 7, 1961 2,997,406 Freeman et al Aug. 22, 1961 FOREIGN PATENTS 1,819 Great Britain Jan. 23, 1897 22,674 Great Britain Oct. 28, 1898 178,853 Great Britain Mar. 1, 1923 

1. IN A PROCESS FOR THE PRODUCTION OF COATED PAPER IN WHICH A TRAVELING PAPER WEB TO BE COATED IS CONTINUOUSLY DRAWN ON A RESILIENTLY SURFACED BACKING ROLL FROM A FIRST PRESSURE NIP FORMED BETWEEN THE RESILIENTLY SURFACED DDRIVEN BACKING ROLL AND DRIVEN COATING APPLICATOR ROLL TO A SECOND PRESSURE NIP FORMED BETWEEN THE SAID BACKING ROLL AND A FLEXIBLE BLADE, THE STEPS OF: (A) DRIVING THE BACKING ROLL AND SAID COATING APPLICATOR IN COUNTER ROTATION AND IN THE DIRECTION OF WEB TRAVEL; (B) TENSIONING THE SAID TRAVELING PAPER WEB IT IS PASSING TO THE BACKING ROLL TO THE EXTENT OF ABOUT 60-80% OF THE WEB STRENGH; (C) WRAPPING THE SAID WEB WHILE SO TENSIONED AROUND THE SURFACE OF SAID BACKING ROLL TO THE EXTENT OF AT LEAST O 90$ CIRCUMFERENTIAL ARC; (D) APPLYING WITH SAID APPLICATOR ROLL AN EXCESS OF A VISCOUS COATING TO SAID WEB AT SAID FIRST PRESSURE NIP AT A POSITION FOLLOWING THE POSITION OF WEB APPLICATION TO SAID BACKING ROLL SURFACE BY CIRCUMFERENTIAL ARC OF AT LEAST 45$ TO FORM A UNIFORM LAYER OF COATING ON 